Highlight "Options" at the main menu and hold L1 + L2 + R1 + R2. This will bring up the "Security Clearance" screen where you'll enter the following codes. Hold X to change a character, and press to enter a code.
Enter 66BAUER as a code.
Enter 62ALMEIDA as a code.
Enter 72DESSLER as a code.
Enter 54PALMER as a code.
Finish a mission with at least 90% completion to unlock pictures, models, and movies for that mission.
Thanks to Revolution reader TYHTER, jackfan!